The invention relates to a light-emitting unit for optical fiber lightguides, especially for signal display devices to display traffic signals, this unit exhibiting, for emitting the light transmitted to a raster dot in the display area by way of lightguides, a light entrance end for connection with the feed end of the associated lightguide, and a light exit end constituting in the display area the light exit surface of the raster dot, as well as a lightguiding central zone between the light entrance and light exit ends, this zone flaring toward the light exit end in the a substantially conical or pyramidal fashion.
Such light-emitting units have been known, for example, from DE 24 37 580 C2, incorporated herein by reference. DE 24 37 580 C2 discloses fiber-optic signal display devices for the emission of light signals, especially traffic signals, wherein the symbol to be displayed is composed of a plurality of light dots in the raster screen of a display area. The illumination of the light dots takes place by means of one or several multi-arm lightguides combined in a bundle shape at one of their ends for simultaneous and joint illumination by means of a light source, the light guides terminating at their other ends into the corresponding apertures of the raster screen. According to DE 24 37 580 C2, these apertures contain light-emitting units consisting of an optically transparent material and exhibiting essentially the shape of truncated cones. The units are arranged in the apertures in such a way that their basal planes form the light-emitting areas of the raster dots in the display area whereas their section planes on the rear side of the raster screen serve for optical coupling to the respectively associated lightguides.
According to DE 24 37 580 C2, the use of substantially truncated, cone-shaped, light-emitting units have the advantage that the radiation characteristic of the light from the individual raster dots is improved to the effect that the radiated light is not only more strongly focused, thus providing a higher luminous density in the direction toward the observer, but moreover the diameter of an individual raster dot is enlarged in comparison with the diameter of the feeding lightguide so that the dot can be more readily perceived by the observer. As a result, a strong luminosity and a high sharpness of contours of the radiated light signals are obtained, which is to lead to a still satisfactory visibility of the signals even in adverse weather conditions, according to the reference.
The emission characteristic of the known lightguide unit is determined essentially by the geometry, i.e., the dimensions of the truncated cone. In any event, the emission cone, however, is not a strictly delineated form but rather exhibits a relatively soft transition toward the rim. The angular energy distribution, plotted in cartesian coordinates, resembles a Gaussian curve rather than a rectangular function.
When installing the conventional signal systems on multiple-lane freeways as well as alongside the road, it is desirable for the emission angle to be wider than required for purely individual track signaling on freeways. However, the disclosed systems have the drawback that the axial light intensity is over-proportionately reduced with increasing emission angle. Thus, for example, with a widening of the emission angle from 6 to 12, the axial light intensity is reduced to 25% of its original value.